BiCMOS current switching circuits are known, in which a signal applied to a CMOS transistor determines a current through a bipolar transistor. For example, current steering digital-to-analog converters may include a plurality of bipolar devices, each bipolar device having a respective CMOS device associated with it, and with the total current drawn by the bipolar devices being determined by the inputs to the CMOS devices.
As an example of this, U.S. Pat. No. 5,548,288 describes a current cell and current switch for use in a current-steering digital-to-analog converter. In the circuit described therein, an output current is determined by the digital signal applied to the circuit inputs.
However, the circuit described therein has the disadvantage that it includes a relatively large number of components and hence that it uses a relatively large area in an integrated circuit.